Most active medical devices are designed to enable data exchange with a “programmer”, which is a term used to refer to an external device variously used to verify the configuration of the device, to read information the device has recorded, to post information to the device, to update the internal software and/or firmware of the device. This data exchange between the medical device and the programmer is typically performed by telemetry, that is to say, by a technique for remote transmission of information without galvanic contact.
Telemetry has most often been performed by an inductive coupling between coils of the implanted device and of the programmer, which technique is known as the “induction method”. Because the coupling requires a very close distance between the coils, this technique has the particular disadvantage of requiring a “telemetry head” connected to the programmer, which contains a coil an operator places in the vicinity of the site where the device is implanted.
It has recently been proposed to implement another non-galvanic telemetry coupling technique, using the two components of an electromagnetic wave generated by transmitter/receiver circuits operating in the radiofrequency (RF) domain, typically in the range of frequencies of several hundred megahertz. This technique, known as “RF telemetry”, allows programming or interrogating medical devices, including implanted devicesm at distances greater than 3 m, and therefore permits the exchange of information without handling of a telemetry head, and even without any external operator intervention. One such active medical device implementing such an RF telemetry is for example described in EP 1862195 A1 and its counterpart U.S. Pat. No. 7,663,451 (both assigned to Sorin CRM S.A.S., previously known as ELA Medical).
The communication protocol between the active device (usually an implanted device) and the base station (i.e., the programmer of a “home monitor” device) is particularly governed by standard EN 301 839 Electromagnetic Compatibility and Radio Spectrum Matters (ERM)—Short range devices (SRD)—Ultra Low Power Active Medical Implants (ULP-AMI) and Peripherals (ULP-AMI-P) operating in the frequency range 402 MHz to 405 MHz. It should be understood, however, that the present invention is not limited to use in the Medical Implants Communication Systems (MICS) band of 402-405 MHz, but is generally applicable to all bands that could be used for RF telemetry, including Industrial, Scientific and Medical (ISM) public unmarked bands 863-870 MHz, 902-928 MHz and 2.4 GHz used by medical devices.
RF telemetry is, however, subject to many disturbances in the electromagnetic environment, including signals from radio, television and mobile telephony, plus many specific parasites that may be produced in the immediate vicinity of the implanted patient (e.g., in hospitals). All these disturbances are likely to cause interferences and disrupt data transmission.
Also, unlike induction method transmissions which have good noise immunity, it is not certain that an RF telemetry transmission can be carried to completion without interruption. If there are too many unrecoverable errors in the transmission, the ongoing transmission process must be abandoned and started over (completely repeated), preferably with new transmission parameters (e.g., another type of modulation (or modulation scheme), reduced transmission data rate, selection of a different channel). In such case, the energy that was spent for the failed communication becomes wasted.
However, the RF telemetry involves relatively high energy consumption, at least at the scale of an implanted device whose autonomy (including its useful life) is an extremely critical parameter. Consequently, multiple interrupted communications can in the long term have a significant impact on autonomy of the device.
For the optimization of transmission, it is important to choose, before starting to send the data, a specific modulation scheme and a data rate level, which determine the communication link performance. These parameters of modulation scheme and data rate (which are collectively called “transmission configuration”) are factors involved in the so-called “link budget”, that is to say to what the device has to run the RF telemetry transmission application.
The link budget is related to the applicable distance over which the data transmitted by the implanted device can be received. It includes the basic parameters of the various functional elements implemented: antenna gain, transmission power, and transmission channel bandwidth (the latter being directly related to the modulation scheme and the data rate used).
In case of limit conditions on the link budget, the good or bad transmission sequence should be monitored, so as to possibly alter the transmission configuration in case of failure before any reiteration of the transmission.
Thus, U.S. Pat. Publication No. 2010/0198304 A1 proposes to evaluate during transmission a metric of quality of service (QoS) and to modify if necessary the modulation scheme based on that assessment. The disadvantage of this method is that it needs to be first confronted with the problem and thereafter react to the problem to change the modulation scheme. The purpose is then to operate an “on the fly” reconfiguration, with a significant time lost between the time the device decides to change the transmission configuration and when the transmission can be restarted with the new configuration.
Indeed, the changes needed are not instantaneous as the clock system has to be reestablished, re-synchronized, as a reset of the transmission packet preamble for hardware synchronization has to be performed. These changes are time consuming and delay the availability of data at the device reception side, which is a serious drawback when the data is critical.
Finally, even after a transmission configuration change, it cannot be ensured that the new transmission configuration is optimal—it is indeed required to wait for a new QoS evaluation factor to know.
Another aspect of the choice of the transmission configuration, besides the greater or lesser probability of failure in a noisy environment, is the possibility of increasing the link budget by reducing the data rates, and thus the bandwidth, and consequently the impact of noise in the band. This reduction provides a gain on the applicable propagation distance (typically, the patient's room) and/or improved immunity against environmental variability for a given distance (typically, the environment of an operation room). But a reduction in the data rate is accompanied of course by a longer transmission duration, which can sometimes be a disadvantage.
The problem recognized to exist by the inventor is to find a technique to directly determine the best transmission configuration among several options, that determination being made a priori before starting to transmit.
Being able to directly use the best available transmission configuration rather than to try several ones before getting to the right one—as in the techniques implementing a QoS metric—is a factor that can provide a significant gain in both (i) reduced energy consumption, and hence an improved life duration of the device, and (ii) availability of information to the recipient (e.g., the remote programmer that retrieves data sent by the device).
This aspect is particularly important in the case of “control/command” communication phases between a programmer and an implanted device: in the latter case, the data rate (transmission speed) is not critical (the size of data is generally not very large), but instead the fact that they are actually received (deterministic reception) is very important.